1. Field of the Invention
This invention relates to a sheet conveying apparatus and a sheet conveying method for separating a plurality of sheets piled for processing such as copying or character reading one by one and feeding them to a processing station.
2. Related Background Art
The apparatus or method of this type is used, for example, in an image forming apparatus or the like capable of forming images multiplexly on both or one surface of a sheet.
Heretofore, in a both-surface image forming apparatus, an intermediate tray for containing therein transfer materials having images formed on first surfaces thereof has generally been provided, and after the termination of the image formation on the first surfaces, the transfer materials piled in the intermediate tray have been separated one by one and fed out therefrom, and have been again conveyed to the image forming station to effect image formation on second surfaces of the transfer materials. An important point in effecting such both-surface image formation is the reliability of the conveyance of the transfer materials, that is, whether the conveyance of the transfer materials can be accomplished properly. Particularly, the problem is the reliability of the re-feeding operation of separating and feeding the transfer materials having images formed on the first surfaces thereof one by one from the intermediate tray.
FIG. 2 of the accompanying drawings shows an example of the both-surface image forming apparatus according to the prior art. In FIG. 2, the reference numeral 1 designates the image forming apparatus body, the reference numeral 2 denotes an original supporting table for supporting an original thereon, the reference numeral 3 designates a photosensitive drum bearing the image of the original, the reference numerals 4, 5, 6 and 7 denote mirrors for forming the image of the original on the photosensitive drum 3 and forming an electrostatic latent image thereon, and the reference numeral 8 designates an imaging lens. The reference numeral 9 denotes a primary charger for uniformly charging the photosensitive drum 3 before imaging, the reference numeral 10 designates a developing device for causing toner to adhere to the electrostatic latent image formed on the photosensitive drum 3 and developing the latent image into a toner image, the reference numeral 11 denotes a transfer electrode for causing the toner image formed on the photosensitive drum 3 to be transferred to a sheet, and the reference numeral 12 designates a cleaner for collecting the toner which has become unnecessary after the transfer.
One of sheets contained in a cassette 106 is taken out by a feed roller 107, is fed out by register rollers 109 at a predetermined timing synchronized with the image on the photosensitive drum 3, and has a toner image formed on a first surface thereof by the transfer electrode 11, whereafter the toner image is fixated by a fixating device 109. When an image is to be again superposedly formed on the same surface of the sheet having an image formed on the first surface thereof, the sheet is directed to a conveyance path 113a by a change-over guide 110 and is received into an intermediate tray 111.
Also, when an image is to be formed on a second surface of the sheet which is opposite to the first surface, the sheet is directed to a conveyance path 113c by the change-over guide 110 and is discharged to a half-way position by discharge rollers 117, whereafter the discharge rollers 117 are rotated in a reverse direction and the trailing end edge of the sheet is directed to a conveyance path 113b by the change-over guide, and the sheet is received into the intermediate tray 111.
Next, when copying is to be effected again on the first surface or on the second surface, sheets each having an image formed on the first surface thereof are re-fed one by one from the intermediate tray 111 and conveyed to the image forming station 107, and copying is effected on the second surfaces thereof. Here, it is sometimes the case with the sheets piled in the intermediate tray after copying has been effected on the first surfaces thereof that silicon oil adheres to the first surfaces thereof by the sheets passing through the fixating device 108 for the fixation of the toner image thereon or warp (curl) is created in the end portions of the sheets by heat or pressure applied thereto during the fixation. This may lead to the occurrence of duplex feed or jam during the re-feeding from the intermediate tray.
FIG. 3 of the accompanying drawings shows an example of a both-surface image forming apparatus constructed so as to enhance such reliability. This example of the prior art is designed such that each time a sheet is placed in the intermediate tray, the entire bundle of sheets placed in the intermediate tray is conveyed little by little by a pair of conveying rollers 114 and the sheets are piled in the form of a staircase in which the sheets deviate little by little from one another.
The group of sheets thus piled in the form of a staircase is collectively conveyed toward a pair of rollers 116 after the termination of the first surface image formation and as soon as the lowermost sheet leaves the pair of rollers 114, the pair of rollers 114 is stopped, whereby only the lowermost sheet nipped between the pair of rollers 116 at this time is conveyed by the pair of rollers 116 to thereby effect the second surface image formation. According to the present example of the prior art, it becomes possible to improve the duplex feed preventing performance during re-feeding.
The technique of piling sheets each having an image formed on one surface thereof in the form of a staircase and re-feeding them to form images on both surfaces of the image is described, for example, in U.S. Pat. Nos. 4,172,655 and 4,573,789.
However, in the above-described example of the prior art, when re-feeding is to be effected, there must be created a condition in which the lowermost sheet to be re-fed leaves the pair of rollers 114 and is nipped by only the pair of rollers 116 and the other sheets are nipped by only the pair of rollers 114. For that purpose, it is necessary to set the distance between the pair of rollers 114 and the pair of rollers 116 to a length slightly shorter than the length of the sheets. However, if the distance between the pair of rollers 114 and the pair of rollers 116 is fixed, sheets of a plurality of sizes cannot be re-fed.
Also, if sheets having images formed thereon are superposed one upon another or such sheets are separated and re-fed, silicon oil, toner, etc. adhering to the sheets rub against each other, and this has led to the problem that the sheets become stained.